Dog owners are urged to be vigilant around stagnant waters following the death of four dogs in Northern Ireland.
The deaths all occurred this week shortly after owners had taken their dogs to Lough Melvin on the Fermanagh/Leitrim border.
Aoife Ferris, a vet with Lakeland Veterinary Services in Belleek, said the pets seen at the veterinary practice displayed symptoms such as drooling at the mouth and seizures.
"There is no treatment for this. It works on the body very fast," she told RTÉ's News At One.
"We would encourage dog owners to keep their dogs away from stagnant waters.
"Blue-green algae is very toxic to pets."
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Ms Ferris added that contamination could occur by swimming in the water, or by the animal licking its paw after coming into contact with the algae.
She said tests were being conducted locally but that algae on the water caused problems to pets in previous years.
The algae, which appears as a blue-green film on the surface of the water, is more prevalent during warm weather.
Ms Ferris said it can also be present as a brownish sludge at the edge of the water.